The SS Booster is part of the Spaceship, which you need to build in order to win a Science victory. Once it's finished, transport it to your capital and use its Add to Spaceship ability.

The booster is usually one of the first parts that becomes available, so start constructing them right away. Unlike the other parts, three units of it must be constructed. Also, be warned that each time you add a part to the Spaceship, all players receive a global alert, which allows them to track your progress. To conceal your activities and avoid making yourself a priority target for more aggressive players, you may want to wait until you've finished multiple parts before you start adding them.

The spaceship's boosters are used to propel the craft off of Earth and into a stable orbit. They are similar to (but much larger than) the simple but reliable and powerful boosters the Russians have been using to put their ships into space for decades. The boosters are ejected during the launch, falling harmlessly into the ocean where they can be retrieved by waiting cargo vessels.